What transaction isolation levels do you know in PostgreSQL?

In PostgreSQL, there are four levels of transaction isolation defined by the SQL standard:

1. Read Uncommitted — the lowest level of isolation. Allows reading data that has not yet been committed (dirty reads). In PostgreSQL, it effectively behaves like Read Committed, as dirty reads are not supported.

2. Read Committed — the default level. A transaction sees only data that was committed at the start of each individual query. Changes made by other transactions may appear between queries within the same transaction.

3. Repeatable Read — a transaction sees a snapshot of the data at the start of the transaction. All queries within the transaction see the same data, even if other transactions modify and commit changes. It prevents non-repeatable reads but allows phantom reads.

4. Serializable — the strictest level of isolation. Guarantees full serialization of transactions, preventing phantom reads and ensuring behavior equivalent to sequential execution of transactions.

Example of setting the isolation level:

BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
-- SQL operations
COMMIT;